The present invention relates to a so-called rear-projection television, in which images on a CRT projection tube are projected from the rear onto a transmission type two-layer screen through a projection lens.
One example of a conventional rear-projection TV using a transmission type two-layer screen is shown in FIG. 5. In the figure, reference numeral 11 denotes a cabinet containing a projection CRT and a projection lens; 12 a Fresnel lens formed with a plurality of concentric circular grooves; and 13 a lenticular lens formed with a large number of vertical grooves. The Fresnel lens 12 and the lenticular lens 13, stacked together and secured to the cabinet, form the transmission type two-layer screen.
The transmission type two-layer screen is so constructed as to be able to provide easy-to-see images when they are projected onto a large-sized screen from the rear, i.e., from the side of the screen opposite the viewer. The grooves to produce the desired effects of the Fresnel lens and the lenticular lens are formed in the facing surfaces of these lenses.
Rays of light passing through the peripheral portion of the screen, remote from the light axis at the screen center, are converged by the Fresnel lens 12 toward the viewer in front of the screen. The lenticular lens 13 widens the horizontal directional characteristic of the screen so that the brightness will be uniform over the entire lateral range of the screen.
To prevent any color misalignment of the image, the Fresnel lens 12 and the lenticular lens 13 are precisely aligned in position before being installed into the cabinet 11 during manufacture and assembly.
With projection TVs using such a two-layer screen, however, there are some drawbacks. Because TV sets are subjected to vibrations during transport, the grooved surfaces of the contacting Fresnel and lenticular lenses are rubbed against each other to form fine wear particles, which adhere to the inside of the screen staining it. When delivered to the user and set up, the rear-projection TV, though a new product, may produce blurred images with reduced brightness and clarity or color misalignment caused by the fine particles in the screen that disturb the projected rays of light.
A common practice to prevent this is to put a protective sheet made of such material as foamed polyethylene between the Fresnel lens and the lenticular lens to protect the opposing surfaces of the two lenses and thereby prevent the formation of fine wear particles.
The manufacturing process for the projection TV of this kind includes adjusting and inspection, such as a projection lens focus adjustment, a CRT projection tube focus adjustment, a convergence adjustment, a white balance adjustment and a final inspection. These adjustment processes require a test image to be projected onto the screen.
However, in the conventional projection televisions with the above-mentioned protective sheet of such material as foamed polyethylene inserted in the screen between the Fresnel and lenticular lenses, a jig screen identical to the product screen must be used during the TV adjustment processes because of the opaqueness of the protective sheet, resulting in an increase in the amount of work and time required for the manufacture.